Asked by Sarah

The front row of a stadium has 25 seats. Each of the other rows has two more seats than the row in front of it. how many seats are there altogether in the first 20 rows?

i know how to do this in the "long" way but how do you do this is a "shorter" way by finding the pattern? Explain please!!
Answered by Steve
The seat count forms an arithmetic sequence

25,27,29...

where a = 25, d=2

The sum of the 1st 20 terms is thus
S20 = 20/2 (2*25 + 19*2) = 880
Answered by Sarah
what equation is that?
Answered by scarlett
S= n/2 (a1 + an)